Preservation of foods and biomaterials by drying is an ancient method still in practice on a large scale. However, it has been only recently recognized that it is not the absolute quantity of water but a combination of the state and amount of water available for microbial and biochemical reactions which determines the nature and extent of spoilage. The availability of water in a system is expressed by its sorption isotherm which is a plot of the amount of water as a function of the relative humidity or activity of water vapor of an atmosphere in equilibrium with the system at a specified temperature. Thus, it is the water activity (A.sub.w), a measure of the available water in a biomaterial, which controls deteriorative reactions.
To achieve extended shelf life for dried foods, it is necessary to determine for each type of material the water activity value at which the least deteriorative reactions occur. This determination requires many experimental studies wherein it is necessary to maintain samples of the materials at a specific water activity value. Although there are three basic methods of obtaining complete sorption isotherms, the gravimetric method is preferred by most investigators for adjustment of water activity of samples to a specified value. In this method test samples are allowed to equilibrate to a preselected water vapor pressure in the headspace around the sample in a sealed equilibration chamber maintained at constant temperature. Theoretically, at equilibrium the water activity of the sample is determined by the relative humidity of its surrounding which in time become the same. However, a true equilibrium is never attained since that would require an infinitely long period of time and the equilibrium process is terminated when the difference between successive weights of the sample becomes less than the sensitivity of the balance used. While various techniques have been suggested to accelerate this approach to equilibrium, depending upon the type of material and the degree of accuracy required, it could still take anywhere from a few days to weeks or even months to achieve the desired result. Unfortunately, the long time required to attain the desired water activity can alter the physico-chemical and microbiological nature of the sample under study. In certain cases deteriorative reactions may proceed to such an extent as to render the quality of test materials far remoted from their initial levels, thus confusing the results.
When reference is made to the relative humidity of the sample, it will be understood that this is done in accordance with conventional nomenclature in which equilibrium relative humidity (ERH) is defined as 100 times the ratio of the vapor pressure in the sample to the saturated water vapor pressure of pure water at the same temperature. The value so obtained is numerically equal to the water activity of the sample expressed as a percentage.
Equilibrium relative humidity of a sample with that of its surrounding is reached by adsorption or desorption of water vapor between the sample and its surrounding atmosphere. The adsorption or desorption of water vapor is a mass transport phenomenon driven by the water activity gradient between the test material and its surrounding. When the water activity of the sample is higher than that of the surrounding, moisture will desorb from it and will go into the surrounding, and vice versa, until an equilibrium is established. As the sorption process proceeds, the driving force continually diminishes and the rate of sorption slows. This process can continue for an infinitely long period of time before true equilibrium is reached.
The need is clear, therefore, for a new, simple and accelerated method of achieving the desired water activity of a sample of biomaterial for comparative research purposes.